Synopsis

Remapping of a quartzofeldspathic gneiss-dominated part of the Lewisian Complex in the NW Highlands of Scotland, originally mapped by the Geological Survey in the latter part of the nineteenth century, has added new information about the lithologies and their distribution, and established a tectonothermal–igneous history. Linking new data for the gneisses of Strath Dionard–Rhiconich to the results of isotopic studies points to the development of their protolith in late Archaean times as the result of arc magmatism similar to that at a modern ocean–continent margin. Compositionally, most of the gneisses, and those southwards towards Laxford, correspond to granodiorite with adakitic affinities. Using a comparison with a well-substantiated model for coeval late Archaean crust formation of part of the Wyoming Craton, they are interpreted as representing a stage in a tectonomagmatic cycle when the products of metasomatism and melting in the mantle were emplaced as sheet-like banded plutons at middle crustal levels. Less common granitic gneisses represent products of further melting in the thickened middle crust. Xenolithic blocks of subducted older sedimentary rocks, carried upwards by the magmas, are now seen as elongate lensoid masses of calc-silicate rock and quartzite that are grossly concordant with the banding in the gneisses. Also generally concordant with this banding are elongate lensoid masses of amphibolite, the protolith of which is considered to have been the product of mafic magmatism during the convergent margin arc activity.

In early Proterozoic times the late Archaean assemblage was extensively reworked. The development of penetrative foliation in both gneisses and amphibolites was associated with isotopic homogenization that, in the southerly continuation of the domain towards the type locality of the Laxfordian orogenic belt at Laxford, has been dated at c. 1860 Ma. This preceded the imposition of the current major structural pattern that included the formation of the Strath Dionard antiform and the parasitic Gharbh Mhòr folds. Abundant granites and pegmatites, subsequently emplaced in an injection complex at Strath Dionard, are linked with the c. 1855 Ma suite of acidic intrusions that are prominent at Laxford. The rock assemblage and sequence of development are like those of other early Proterozoic mobile belts formed as the result of extensive reworking of late Archaean cratons. Limited mineralogical and structural modification of the ≥1850 Ma gneiss–amphibolite–granite assemblage, and pegmatite emplacement associated with (micro)-plate collision, took place during the development of the c. 1750 Ma Calvian orogenic belt. Further limited modification and pegmatite emplacement was at c. 1670 Ma.

The structural–metamorphic–igneous histories described, integrated with isotopic data, do not support interpretations proposing a transition between the Strath Dionard–Rhiconich (–Laxford) gneiss–amphibolite domain and the adjacent granulite–dolerite dyke domain; or that the quartzofeldspathic gneiss–amphibolite domains in some other parts of the NW Highlands can be correlated with the type area of the Laxfordian orogenic belt.

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